President Donald Trump has the authority to launch a strike against North Korea without congressional approval, Sen. Lindsey Graham (R-S.C.) said Thursday.

“There’s nothing in the Constitution limiting the ability to use force to protect America,” Graham told conservative radio host Hugh Hewitt, as the president ramped up his war of words with Pyongyang.

Trump — who promised “fire, fury and frankly power the likes of which this world has never seen before” if North Korea continues its provocations — on Thursday responded to Kim Jong Un’s threats to strike Guam by saying he had perhaps not been strong enough in his remarks on the isolated nation in his earlier comments.

“I think that’s the first time [North Korea] heard it like they heard it,” he said. “And frankly, the people that were questioning that statement, was it too tough, maybe it wasn’t tough enough. They’ve been doing this to our country for a long time. For many years. And it’s about time someone stuck up for the people of this country.”

The barb-trading has brought the U.S. and North Korea to the brink, raising the specter of possible nuclear war — the results of which would likely prove devastating. While Secretary of State Rex Tillerson has downplayed the tense situation and said Americans should “sleep well,” recent polls show Americans are worried about a potential conflict with North Korea — and would vastly prefer a diplomatic solution to the standoff over military action. They join more than 60 members of Congress, who on Thursday called for “caution and restraint” on North Korea.

Still, Trump — whose administration is engaging in backchannel negotiations with Pyongyang — continued to talk tough on the country, tweeting that the U.S. military was “locked and loaded” for a possible strike.

So, can Trump proceed without Congress?

President Donald Trump speaks on North Korea during a security briefing Thursday in Bedminster, New Jersey.

Stephen Vladeck — a professor of law at the University of Texas School of Law — told CNN at the time that while the Supreme Court has not definitively settled the question of when, if ever, presidents can use military force without Congressional authorization, the legality of Trump’s Syria strike was especially uncertain.

A consensus of scholars, he told CNN, agree that presidents can take action without Congressional approval in matters of self-defense, non-combat missions and scenarios in which Congress has given general authorization. The April strike, Vladeck argued, didn’t easily fit into those three categories.

A potential attack on North Korea, Vladeck said in an email Friday, could be just as legally unclear — and depend on whether a hypothetical strike by Trump was offensive or in response to an attack.

“The Constitution has been interpreted to distinguish between offensive uses of force (which require legislative support) and defensive uses of force (which do not),” Vladeck said. “But there’s a lot of gray area as to where the line is between offense and defense — and whether pre-emption could ever be defensive.”

Congress is authorized to declare war under Article 1, Section 8 of the U.S. Constitution. But whether the president can conduct military action without such authorization has long been a matter of debate.

Presidents, perhaps unsurprisingly, have on numerous occasions asserted their ability to bypass Congress to use the military.

Former President Barack Obama speaks in the Rose Garden on the death of Libyan leader Moamer Kadhafi in October 2011.

He argued in 2011 that he did not need a go-ahead from Congress to conduct military action in Libya, which some said violated both the Constitution and the War Powers Resolution.

Under that 1973 resolution, Obama was required to report any military action to Congress within 48 hours of taking it and get approval for it within 60 days. While he did notify Congress of his action in Libya, his team said they could continue their action in the North African nation indefinitely with or without Congress.

“Allowing the trivialization of the War Powers Act to stand will open the way for even more blatant acts of presidential war-making in the decades ahead,” Bruce Ackerman, the Sterling Professor of law and political science at Yale University, wrote in the New York Times in 2011.

But Professor Philip Bobbitt — director for the Center for National Security at Columbia Law School — argued in Foreign Policy in 2013 that “there is ample precedent in recent history ... to support the view that the president does not need a joint resolution of Congress, much less a declaration of war, in order to initiate hostilities on a valid constitutional basis.”

Further, Bobbitt wrote, it would not be strategically sound to force the president to obtain Congressional approval before all military actions.

“In the current strategic context, does it make practical sense to require a joint resolution of Congress before the president can act?” Bobbitt asked. “For example, with respect to extended deterrence — the vow to treat an attack on our allies as an attack on ourselves — I would think the answer is clearly no.”

A man watches a television screen showing President Donald Trump and North Korean leader Kim Jong Un during a news program at the Seoul Train Station in South Korea Thursday.

Still, there have been multiple attempts to prevent presidents from taking military action without Congressional approval. In 2014, Sen. John McCain (R-Ariz.) and Sen. Tim Kaine (D-Va.) sought to reform the 1973 War Powers Resolution by forcing a “clear political consensus” between the president, Congress and the American people before combat actions are undertaken. More recently, in January 2017, Rep. Ted Lieu (D-Calif.) introduced a bill prohibiting the president from conducting a “first-use nuclear strike unless such strike is conducted pursuant to a congressional declaration of war expressly authorizing such strike.” That bill, though, is currently sitting in the House Committee on Foreign Affairs.

The issue, Vladeck said, is one difficult to paint in “black and white.” But, he said an attack from North Korea “would change the calculus” — and make Congress more likely to give him the go-ahead, anyway.

“If [Kim Jong Un] utters one threat in the form of an overt threat,” Trump said Friday, “or if he does anything with respect to Guam or anyplace else that’s an American territory or an American ally, he will truly regret it and he will regret it fast.”

“I hope that they are going to fully understand the gravity of what I said, and what I said is what I mean … those words are very, very easy to understand.”